Silicon nitride bodies having a density less than theoretical are porous enough to permit the migration of oxygen therein. When such porous bodies are exposed to a high operating temperature, for example, at a temperature of about 1900.degree.F, some of the silicon nitride article oxidizes to produce silicon dioxide which expands upon cooling and causes a weakening of the structure.
If a silicon nitride article having a density in the range of 69 to 82 percent theoretical is oxidized for a period of 200 hours at a temperature of 1900.degree.F, the strength of the material can be reduced by as much as 50 percent. Because of this oxidation problem associated with less than theoretically dense silicon nitride, the material generally would not be suitable for use in forming portions of a gas turbine engine designed to operate at high temperatures.
It is an object of this invention to provide a method of increasing the oxidation resistance of a silicon nitride article. It is a further object of this invention to provide an economical method for treating silicon nitride articles in order to increase their useful life and oxidation resistance at high temperatures so that the silicon nitride article may be used in high temperature environments.